chore: release v0.20.0 (#155)

Pick several issues and change them at once, in lists, tables and on the board; archived issues
weigh much less on each device; and two security fixes. Updating a self-hosted copy needs no
migration and no new setting. The protocol goes to version 25, so open tabs reload and every
browser starts again from a fresh copy of its workspaces. ([#148], [#149], [#150], [#151],
[#153], [#154])

Security

• An attached file's upload belongs to whoever attached it. The browser links a file before its
  upload finishes, so another member could see its id; the first upload to an id now wins
  atomically, and anyone else's upload to it is refused instead of being taken as done. ([#150])
• Leaving a workspace, and deleting a user, now tell the workspace at once, so the person's open
  tabs lose access as they do when an admin removes them. Mail to a request's reply address is
  checked against the request and its sender before it counts against any limit or stores any
  attachment. ([#153])

Added

• Bulk edit: pick issues by a checkbox, X, Shift or Ctrl/Cmd+A in lists, tables and on the board,
  then set their status, priority, assignee, labels, project or cycle, archive the closed ones,
  or delete them, from a bar at the bottom. Each change is the same one a single issue's menu
  makes, so history, the inbox and webhooks see it as usual. ([#148], [#151], [#154])
• Track has its own icon: an issue in progress, on the dark tile of the new Gateweave mark.
  ([#152])

Changed

• Archived issues stay in each browser's copy only as a slim row: key, title, status, team,
  project, cycle, parent and estimate. Lists, boards, the archive page, links, progress and search
  by key or title work from it, offline too; an archived issue's description, comments and
  history need a connection. JSON exports list archived issues apart, with what is known of them.
  ([#149])